This invention relates to flame holders for use in gas turbine engine afterburners.
In military aircraft engines operating with afterburners to enhance thrust, large unsteady pressure oscillations termed screech can occur under some conditions when unsteady heat release couples with the acoustic pressure fluctuations. Screech if not suppressed, can result in instantaneous disintegration of the afterburner hardware such as flameholder, fuel injector, liner and so on. Conventionally acoustic liners are used to suppress screech. The liner has small holes which act as Helmholz resonators and absorb the energy of the unsteady pressure fluctuations. This method suffers from a number of drawbacks: (1) It is costly since the pattern of holes in the liner and their size determine the modes and frequencies of the oscillations absorbed effectively by the liner, and these modes and frequencies cannot be predicted beforehand for a new configuration; (2) the liner has to be cooled and therefore degrades the performance of the afterburner and the efficiency of the engine; and (3) the liner is ineffective at low frequencies.
Current afterburners use one or more concentric annular rings of V-shaped members, sometimes referred to as gutters or flameholders, as flame stabilizers. The flameholders are about 11/2"-2" wide and are about 11/2"-2" deep. The enclosed half-angle of the typical flameholder is generally about 20-24 degrees. The overall blockage to gas flow in the afterburner region offered by the flameholders is approximately 25%. The fuel is sprayed upstream of the flameholder. The flame is established at the downstream lips of the flameholder and is sustained by the recirculating products in the wake of the flameholder. The combustion takes place downstream of the flameholder and is generally unsteady. Under certain conditions the unsteady heat couples with acoustic pressure fluctuations in the afterburner cavity resulting in screech. The screech is generally at high frequencies such as 500-3000 Hz, however, sometimes lower frequency longitudinal modes (100-500 Hz.) are also observed.
The present inventors recognize that the primary mechanism responsible for screech is the interaction between the vortices (spanwise), i.e., the axes of the vortices are transverse to the flow direction, shed at the lips of the flameholder. As these vortices travel downstream they entrain hot recirculating products, pair up and couple with each other. After a time delay, depending on the fuel, velocity and so on, the vortices burn and release heat which in turn affects the dynamic pressure field in the afterburner cavity. The resulting pressure fluctuations at the lips of the flameholder create additional vortices and the process repeats. If the frequency at which this process occurs matches an acoustic mode of the device (depending on the geometry) coupling occurs and screech develops. The vortices, however, do serve the purpose of mixing the cold reactants with the hot products and are therefore vitally important in the sustenance of the flame. The flameholders are therefore essential in the afterburner. The problem is how to alleviate screech, i.e., eliminate the need for the costly acoustic liner while at the same time reducing screech to acceptable levels.
It is an object of the present invention to provide a flameholder which could replace flameholders currently used in aircraft engine afterburners and suppress combustion-induced screech.